Based on the new class of superconductors as described in the article by Bednorz and Mueller, entitled "Possible High-T.sub.c Superconductivity in the Ba-La-Cu-O System", Zeitschrift fuer Physik B, Condensed Matter, Vol. B64, 1986, p. 189-193, different field-effect transistor (FET) structures have been developed and published using a high T.sub.c (transition temperature) superconductors. FET structures with superconducting channels have been described in the following articles: "Superconducting Field-Effect Transistor" by F. F. Fang et al., IBM Technical Disclosure Bulletin, Vol. 19, No. 4, September 1976, pp. 1461-1462, and "Experimental Considerations In the Quest for a Thin-Film Superconducting Field-Effect Transistor" by A. F. Hebard et al., IEEE Trans. on Magnetics, Vol. MAG-23, No. 2, March 1987, pp. 1279-1282. In these articles, structures are described with a superconducting channel having a thickness of about 10 nm. An electric field generated by applying a voltage between a gate electrode and the superconductor causes a slight change in carrier density in a thin surface layer of the superconductor. This change in carrier density in turn results in a shift in T.sub.c in the thin layer. By applying signals to the gate, the thin layer can be switched between "superconducting" and "normal conducting" states. This results in a change in the channel resistance.
Since the field induced effect does not extend deeply into the channel material, various approaches to enhance the magnitude of the effect have been published by A. T. Fiory and A. F. Hebard in the following two articles: "Field-Effect and Electron Density Modulation of the Superconducting Transition in Composite In/InOx Thin Films", Physica 135B, 1985, pp. 124-127, North-Holland, Amsterdam, and "Electric Field Modulation of Low Electron Density Thin-Film Superconductors", Proc. Internat. Workshop on Novel Mechanism of Superconductivity, Berkley, June 1987. The first cited article of Fiory and Hebard relates to the fabrication and measurement of a superconducting MISFET. Another article on this subject, relating to a field effect in strontium titanate, has been published by M. Gurvitch et al. "Field Effect on Superconducting Surface Layers of SrTiO.sub.3 ", Materials Research Society, 1986, pp. 47 -49.
The disadvantages of these surface effect devices is that the change in channel resistance is quite small. Even in the switched thin surface layer the change is only from metallic conductivity to superconductivity and, in addition, the bulk section of the channel, not being affected by the applied field, acts as a metal shunt parallel to the thin surface layer. Therefore, the output signals are too small to be able to drive FET devices. Another drawback is that the field induced change in T.sub.c is rather small, i.e., operating temperature, T.sub.op, requirements are stringent. The reason for this drawback is that in order to operate properly, the T.sub.c of the thin layer has to be shifted from a value above T.sub.op to a value below T.sub.op.
Another field-effect transistor is described in European Patent Application EP-A 0 324 044, "A Field-Effect Device with a Superconducting Channel". The channel of this device is about 1 nm thick and consists of a high- T.sub.c metal-oxide superconductor. Since the channel is extremely thin, it can be affected by an electric field without leaving a metal shunt. With a few volts applied to the gate, the entire channel is depleted of charge carriers whereby the channel resistance can be switched between "zero" (undepleted, superconducting) and "high" (depleted). Unfortunately, studies of such devices have shown that, in the suggested configuration, the ultrathin superconducting layers readily degrade during deposition of the insulating layer and the top electrode.
The problem of degradation during the deposition of insulator and electrode can also be avoided by changing the structure in such a manner that the superconducting film is deposited after the insulating layer, and the gate electrode is located underneath the insulator and the superconductor. An exemplary cross-section of such an inverted MISFET is shown in FIG. 1. The gate electrode 10 consists of a conducting niobium-doped strontium titanate, the insulating layer 11 consists of undoped strontium titanate and the current channel 12, which is grown on top of this layer 11, consists of YBa.sub.2 CU.sub.3 O.sub.7-.delta.. Source and drain contacts 13, 14 are situated at the upper surface of the current channel 12 and the gate metalization is fixed at the niobium-doped strontium titanate substrate 10.
Measurements with these and similar structures have shown lower capacitances between gate and source (C.sub.GS) than theoretically expected. In addition only a few percent resistivity modulation was observed. The fact that the performance of the device, is influenced by some degradation at the surface of the doped SrTiO.sub.3 has been published in the article "Electric Field Effect on Superconducting YBa.sub.2 Cu.sub.3 O.sub.7-.delta. Films", by J. Mannhart et al., Zeitschrift fuer Physik B, Condensed Matter 83, pp. 307-311, 1991.
Additionally, measurements of the resistivity and the capacitance of known inverted MISFETs with high- T.sub.c superconducting channel have shown drawbacks of these devices. Such drawbacks seem to be caused by a degradation at the surface of the Nb-doped strontium titanate. Similar effects have been observed and reported by H. Hasegawa et al. in their article "Contact between High-T.sub.c Superconductor and Semiconducting Niobium-Doped SrTiO.sub.3, Japanese Journ. of Appl. Phys., Vol. 28, No. 12, December 1989, pp. L2210-L2212. The authors interpreted their measurement of an Er-Ba-Cu-O thin film superconductor deposited on a Nb-doped SrTiO.sub.3 substrate as an indicator that there are unknown interfacial layers between the high- T.sub.c superconductor layer and the substrate. Further investigations and measurements have shown that a degradation of the doped SrTiO.sub.3 reduces its conductivity and lowers the dielectric constant of the insulating layer. A low dielectric constant of the insulating layer of a field-effect transistor has a negative influence on several parameters of these devices. For example, the change in drain current, resulting from the applied field, the output admittance, and the transconductance are proportional to the dielectric constant of this insulator.